1. Field of the Invention
The invention relates to a pneumatic cylinder with a self-adjusting end position damping arrangement and having a cylinder housing in which is arranged a movable cylinder piston which is acted on at one side by a working pressure, and in which a damping volume which is delimited from the non-pressurized side of the cylinder piston is formed in the region of the end position of the cylinder piston as a result of the movement of the cylinder piston, and to a method for self-adjusting end position damping.
2. The Prior Art
In hydraulic or pneumatic cylinders, an end position damping arrangement is often used in order to prevent the piston from impacting, in the end position, against the cylinder housing or against a stop. It is accordingly an aim of the invention to reduce the speed of a moved mass (piston+load), whose centre of gravity generally lies in the cylinder axis, to a level at which neither the cylinder nor the machine in which the cylinder is installed is damaged or adversely affected by shocks which are generated.
In a known pneumatic cylinder which includes a movable cylinder piston 22 that divides the interior of the cylinder into a first variable-volume chamber 40 containing a piston fluid at pressure (p1) and a second variable-volume chamber 50 containing an end position damping arrangement, as per FIG. 1, at the end of the working stroke, in the region of the end stop of the cylinder piston 22, a damping piston 18 is guided by means of a damping seal 21 in a recess 23 of the cylinder piston 22 (indicated by dashed lines), as a result of which an additional chamber is created in the non-pressurized cylinder side —the damping volume 19. The damping volume 19 which is now generated can escape only via a provided valve needle (not illustrated here), for example in the cylinder cover 4. When the cylinder piston 22 retracts, the air which collects in the chamber is compressed and, as a result of the movement of the piston, conducted past the valve needle. The volume however cannot be discharged in the same amount of time as it takes for the piston to retract, as a result of which a pressure rise occurs in said chamber. The piston is retarded by means of said pressure and, in this way, should not impact against the cylinder cover or an end stop but rather should retract slowly with the time-delayed escape of the air. Here, the valve needle is adjusted when the cylinder is first operated. This form of end position damping can be found in many pneumatic or hydraulic cylinders which have an end position damping arrangement such as for example in a piston-rod-less pneumatic cylinder 1 as illustrated in FIG. 1. The disadvantage of said end position damping arrangement is that, as a result of the fixed adjustment of the valve needle, only a certain amount of kinetic energy can be dissipated. A change in the mass m to be damped (for example because the load changes) and/or in the speed v of the piston would require a renewed adjustment, which is however not always possible or is complex in practice. As a result of the complex interaction of mass, speed and end position pressure, the adjustment of the end position damping arrangement is time-consuming and, as a result of the high shock loading in the region of the mechanical end stop, not ideal. This reduces the service life of the cylinder. Furthermore, the adjustment is often carried out inadequately or is even forgotten entirely; this also results in oscillations in the end position and therefore lengthened cycle times.
Also known, for example from EP 949 422 A1, are travel-dependent end position damping arrangements which vary a discharge air cross section as a function of the piston position and can therefore predefine a progressive damping profile. Said damping profile is however dependent on the fixedly predefined geometry and can therefore be optimal only for a certain combination of mass and speed. If the pneumatic cylinder is operated outside the optimum operating point, for example if the working pressure (and therefore the speed) changes or if a different load is moved, the damping is no longer optimal. However, precisely this is the case in practice, since it has been found that the positions at which the pressure peaks occur are different depending on the loading and speed.
Likewise known, for example from DE 37 40 669 A1, are pneumatic shock absorbers with an outlet valve via which the air which is compressed during a damping movement of the piston is discharged. For this purpose, a valve plunger is preloaded by the working pressure and a spring force. If the force of the compressed air exceeds the preload, the outlet valve opens abruptly and the compressed air is expanded via a throttle. In order to be able to operate a shock absorber of said type optimally, a controller is provided, by means of which the pilot pressure counter to which the piston is moved is controlled as a function of the position of the piston. With control of said type, it is possible to obtain self-adjusting damping, but only with a high level of control expenditure.
The present invention is based on the object of specifying an end position damping arrangement of a pneumatic cylinder, and an associated method, which adjusts automatically to different operating parameters, such as for example mass, speed and working pressure, in order to obtain optimum damping within a wide range, and which is of simple and cost-effective design.